Re: [Qemu-devel] [PATCH v4 2/2] block: Support GlusterFS as a QEMU block backend

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Fri, Aug 03, 2012 at 03:57:20PM +0000, Blue Swirl wrote:
> >> > +static void gluster_finish_aiocb(struct glfs_fd *fd, ssize_t ret, void *arg)
> >> > +{
> >> > +    GlusterAIOCB *acb = (GlusterAIOCB *)arg;
> >> > +    BDRVGlusterState *s = acb->common.bs->opaque;
> >> > +
> >> > +    acb->ret = ret;
> >> > +    if (qemu_gluster_send_pipe(s, acb) < 0) {
> >> > +        error_report("Could not complete read/write/flush from gluster");
> >> > +        abort();
> >>
> >> Aborting is a bit drastic, it would be nice to save and exit gracefully.
> >
> > I am not sure if there is an easy way to recover sanely and exit from this
> > kind of error.
> >
> > Here the non-QEMU thread (gluster thread) failed to notify the QEMU thread
> > on the read side of the pipe about the IO completion. So essentially
> > bdrv_read or bdrv_write will never complete if this error happens.
> >
> > Do you have any suggestions on how to exit gracefully here ?
> 
> Ignore but set the callback return to -EIO, see for example curl.c:249.

I see the precedence for how I am handling this in
posix-aio-compat.c:posix_aio_notify_event().

So instead of aborting, I could do acb->common.cb(acb->common.opaque, -EIO)
as you suggest, but that would not help because, the thread at the read side
of the pipe is still waiting and user will see the read/write failure as hang.

[root@bharata qemu]# gdb ./x86_64-softmmu/qemu-system-x86_64
Starting program: ./x86_64-softmmu/qemu-system-x86_64 --enable-kvm --nographic -m 1024 -smp 4 -drive file=gluster://bharata/test/F16,if=virtio,cache=none
[New Thread 0x7ffff4c7f700 (LWP 6537)]
[New Thread 0x7ffff447e700 (LWP 6538)]
[New Thread 0x7ffff3420700 (LWP 6539)]
[New Thread 0x7ffff1407700 (LWP 6540)]
qemu-system-x86_64: -drive file=gluster://bharata/test/F16,if=virtio,cache=none: Could not complete read/write/flush from gluster
^C
Program received signal SIGINT, Interrupt.
0x00007ffff60e9403 in select () from /lib64/libc.so.6
(gdb) bt
#0  0x00007ffff60e9403 in select () from /lib64/libc.so.6
#1  0x00005555555baee3 in qemu_aio_wait () at aio.c:158
#2  0x00005555555cf57b in bdrv_rw_co (bs=0x5555564cfa50, sector_num=0, buf=
    0x7fffffffb640 "\353c\220", nb_sectors=4, is_write=false) at block.c:1623
#3  0x00005555555cf5e1 in bdrv_read (bs=0x5555564cfa50, sector_num=0, buf=
    0x7fffffffb640 "\353c\220", nb_sectors=4) at block.c:1633
#4  0x00005555555cf9d0 in bdrv_pread (bs=0x5555564cfa50, offset=0, buf=0x7fffffffb640, 
    count1=2048) at block.c:1720
#5  0x00005555555cc8d4 in find_image_format (filename=
    0x5555564cc290 "gluster://bharata/test/F16", pdrv=0x7fffffffbe60) at block.c:529
#6  0x00005555555cd303 in bdrv_open (bs=0x5555564cef20, filename=
    0x5555564cc290 "gluster://bharata/test/F16", flags=98, drv=0x0) at block.c:800
#7  0x0000555555609f69 in drive_init (opts=0x5555564cf900, default_to_scsi=0)
    at blockdev.c:608
#8  0x0000555555711b6c in drive_init_func (opts=0x5555564cc1e0, opaque=0x555555c357a0)
    at vl.c:775
#9  0x000055555574ceda in qemu_opts_foreach (list=0x555555c319e0, func=
    0x555555711b31 <drive_init_func>, opaque=0x555555c357a0, abort_on_failure=1)
    at qemu-option.c:1094
#10 0x0000555555719d78 in main (argc=9, argv=0x7fffffffe468, envp=0x7fffffffe4b8)
    at vl.c:3430